A Density Functional Study of the Methanol Synthesis at an Oxygen Vacancy on the Polar ZnO(0001̅) Surface

Abstract

The elementary steps of the methanol synthesis reaction at oxygen vacancies on the polar ZnO(0001 j ) surface have been studied with density functional theory. We report results for reaction and activation energies for the most important elementary steps of the methanol synthesis reaction on this surface. At these oxygen vacancies the barriers for heterolytic cleavage of H 2 into protons, bound in surface OH groups, and hydrides, trapped in the vacancies, is low. The stabilized hydride facilitates the reduction of CO to formyl (stabilized in the vacancy) with a low barrier j100 kJ/mol. After a fast rearrangement to formaldehyde and hydroxymethylene species, a second reduction step leads probably to a surface-bound methoxide which at reducing conditions desorbs from the surface under regeneration of an oxygen vacancy. Beside formate, which appears as a side product, and formyl species, oxygen-bound hydroxymethylene might be another observable intermediate of the methanol synthesis reaction at oxygen vacancy sites on the ZnO(0001 j ) surface.

Prediction of vibrational frequencies of possible intermediates and side products of the methanol synthesis on ZnO($000\overline{1}$0001¯) by ab initio calculations

“…7 For the construction of our systems we started from the wurtzite structure of ZnO with experimental 20-23 lattice parameters a = 3.2495 Å, b = 5.2069 Å, and u = 0.3825 Å. The ideal ZnO(0001) surface can be obtained by cutting this ideal bulk structure orthogonal to the c axis.…”

“…3,4 Hence, different reaction pathways have been suggested assuming oxygen vacancies on ZnO(0001) as the active center and details of the possible mechanisms have been discussed controversially in recent literature. [5][6][7][8][9] Although syngas, generated by steam reforming, consists mainly of CO and H 2 , already due to the Boduard equilibrium and the water gas shift reaction, there is always a small amount of carbon dioxide in the gas phase of the real catalytic system. Furthermore, syngas obtained from biomass, which can also be used for methanol synthesis, 10 might differ in its composition and have a higher CO 2 content.…”

“…They are accessible through the reduction of adsorbed CO molecules by a simultaneous transfer of a proton and an electron pair to the adsorbate. It has been shown that a formyl species at an F 2+ center is less favourable than formyl species at F centers 7 where the carbon atom is bound to a surface zinc atom. So far, formyl species could not be observed experimentally as intermediates in the methanol synthesis, but this might be a consequence of the short lifetime of these species, which is supported by the predicted low reaction barriers to subsequent intermediates.…”

“…So far, formyl species could not be observed experimentally as intermediates in the methanol synthesis, but this might be a consequence of the short lifetime of these species, which is supported by the predicted low reaction barriers to subsequent intermediates. 7 To assist the experimental identification of such formyl species we computed at F 2+ and F vacancies their structures, enthalpies of formation, and vibrational frequencies.…”

An ethyl acetate sensor utilizing cataluminescence on Y2O3 nanoparticles

“…Concerning the pathways involved, a wide variety of proposed reaction intermediate species have been identified via experiment 20, 46–56 and theory 19, 57–61, which certainly is an incomplete list. Although the basic steps in the reduction process via a stepwise hydrogenation of CO are mechanistically rather simple on paper the sheer number of the obtained (and partially conflicting) results suggests that effectively several reaction channels might be accessible for methanol synthesis when taking place over the heterogeneous catalyst.…”